1. Field of the Invention
The invention is directed to water-based inks for ink-jet printing on edibles, to methods of ink-jet printing on edibles using the inks, and to edible products made by such methods. The inks and methods described herein may be employed for forming printed edible products as diverse as pharmaceutical pills and tablets to sausage casings. However, the invention has particular utility for printing on confectionery, including, without limitation, chocolate bars and tableted products, jelly beans, toffees and chewing gums, and particularly for printing high resolution and high definition images on non-planar, non-porous hydrophobic surfaces of polished sugar shell confectionery, such as the surfaces of M&M's® Milk Chocolate and Peanut Chocolate Candies.
2. Description of the Related Art
It is known to print identifying or decorative images on edibles. The present technology for printing on M&M's® Milk Chocolate and Peanut Chocolate Candies is by a contact printing process utilizing an offset roller, in what is referred to herein as rotogravure printing. The rotogravure system is limited in the number of colors that can be applied to a substrate. Traditionally, one color is printed, and with modifications two or three colors may be applied, but full color printing on edibles is not possible. The rotogravure roller is also limited in the surface area of a non planar piece that it can print. As with other contact methods of printing, there is a danger of the rotogravure roller crushing the edible articles being printed. Print designs cannot be changed or modified easily using the rotogravure method, because each new design must be engraved on a roller. If a non-contact printing method such as ink-jet printing could be used, there would be distinct advantages.
The ink systems used in rotogravure printing, generally comprise shellac, ethyl alcohol and pigments, to which may be added dyes, plasticizers, additional solvents and other ingredients to modify the characteristics of the ink. Such inks have a relatively high viscosity such that they sit on the non-porous surface until dry, and they are not ink-jettable. Thus, the inks conventionally utilized for printing on small pieces of confectionery or on pharmaceutical tablets and the like by the rotogravure method cannot be adapted to ink-jet processes.
Another popular method for printing on edibles, particularly on cakes and other large substrates, involves using an edible transfer sheet which can be handled in a printer and which are relatively porous and hydrophilic and therefore can readily accept an image from a water-based ink, including an ink-jetted image. Various methods are then used to transfer the image from the sheet to an edible substrate. An exemplary ink for use with this type of system is disclosed in U.S. Patent Application Publication US 2002/0008751 and comprises water, isopropyl alcohol, sodium lauryl sulphate and FD&C colorant. In the transfer sheet technology, printing is not performed directly on the surface of an edible conveyed past a printhead. Using a transfer sheet requires a component of the substrate to partly dissolve the sheet or to allow the sheet to adhere to the substrate. Thus, the transfer sheet technology is not readily adapted to the high speed production of images on non-planar surfaces of confectionery pieces. The water-based inks adapted for use with transfer sheets do not perform properly for ink-jet printing on non-planar, non-porous and hydrophobic surfaces, as they adhere poorly, dry too slowly and lack opacity.
Ink-jet printing on edibles, particularly on sugar shell confectionery pieces, if the technology could be perfected, would be attractive from many vantage points: it would eliminate the need to contact the edible substrate with a contact member such as a roller. Further, since ink-jet printing is a non-contact printing system, slight variations in the size of edibles would not negatively impact upon printing quality, as typically occurs with pad or roller based systems. Also, an ink-jet printer image is stored as data, and not fixed on a contact member. Thus, if an ink-jet printer could be effectively used, images could be selected, altered, transmitted, and the like, more easily than in contact printing, permitting faster changeover of designs. Use of ink-jet technology would also permit full color printing using multicolor printheads.
Ink-jet printing systems are broadly divided into continuous jet, and drop-on-demand (also called “impulse”) systems in which droplets are generated as needed for ejection to the substrate surface for image formation. Methods of ink-jet printing on edible substrates using continuous jet technology have been disclosed. Most of these are directed to labelling and similar applications which do not require high resolution.
In continuous jet systems, ink is emitted in a continuous stream under pressure through at least one nozzle. The stream is broken up into droplets at a fixed distance from the orifice, typically by a piezoelectric crystal, which is vibrated at controlled frequency adjacent to the ink stream. The function of the piezoelectric crystal is different in a continuous jet system than in a piezojet drop-on-demand system. In a continuous jet system, the ink stream is generated under pressure in the reservoir, and the crystal serves simply to break up the droplets. To control the flow of ink droplets, the inks are electrostatically charged (by addition of salts and other conductive agents) and the droplets are passed through an electrostatic field, which adjusts the trajectory of the droplets, in accordance with digital data signals. The conductance requirements for continuous jet inks therefore range from 700 to 2000 micro Siemens, which generally requires that the ink contains a conductivity enhancing salt. The droplets are either directed back to a gutter for recirculation or to a specific location on the substrate to create the desired character matrix. A typical resolution for a continuous jet printer image in an industrial setting, using a single printhead and a single pass printing is about 75-100 dots per inch (dpi).
Most of the inks developed for continuous jet systems are based on the solvent methyl ethyl ketone (MEK) and are therefore not suitable for use in an edible ink. The inks that have been developed for continuous jet printing on edibles, or on surfaces which may contact edibles, have been pigmented, or based on solvents associated with other drawbacks.
U.S. Pat. No. 5,453,122, for example, discloses an ink which it is said can be applied directly to a foodstuff, or to packaging closely associated with a foodstuff, and which avoids the use of methyl ethyl ketone. The ink is also said to be suitable for use in a piezojet printhead. However the ink relies on the presence of substantial amounts of acetone solvent. If such an ink is less toxic than an MEK-based ink, the substantial amounts of acetone in such an ink limit its use for printing on edibles. Inks having substantial amounts of acetone are less desirable to work with in this area.
U.S. Pat. No. 5,800,601 discloses pigment-based inks said to be suitable for printing on edibles, including glazed candies, using a continuous jet system. Specifically, the disclosure is directed to modification of pigment particle surface characteristics to improve the drying time and adhesion of the inks. These inks are also characterized by the presence of conductivity enhancing salts.
U.S. Pat. No. 5,637,139 discloses non-aqueous inks, also for continuous jet systems, and focused on applications for labelling fruit and the like substrates with citrus #2 dye.
Continuous jet inks also have a very narrow range of acceptable viscosity. Inks having a viscosity above about 10 centipoise (cp) at low shear rates cause the pumps in the printhead to cavitate during use. Below a viscosity of about 2 to about 3 cp, the jets are not stable. Thus most, if not all, continuous jet ink jet inks have a viscosity of about 2.8 to about 6 cp.
Commercially available continuous jet inks have not been developed for printing on hydrophobic edible substrates. The presence of conductivity enhancing salts required in these inks would also be likely to affect taste.
Of the drop-on-demand systems, the most economically important today are piezojet and bubblejet (sometimes referred to as thermal ink-jet) systems. In bubblejet systems, a bubble is formed by a resistance heater in an ink reservoir. The resulting pressure wave from the bubble forces ink through the orifice plate, and as the heat is removed, the bubble begins to collapse and a droplet is ejected. Bubblejet printheads dominate the home and office ink-jet printer markets and they are capable of very high resolution. However, several considerations limit their use for printing on edibles in an industrial setting.
This viscosity of bubblejet inks is very low, on the order of 1.5 cp, necessary so that a bubble can form quickly upon application of minimal voltage to the resistance heater. The known bubblejet printers cannot readily handle the presence of binders or polymeric additives. Moreover, the ink must be capable of withstanding the temperature cycling encountered inside the printhead. For these reasons, the number of inks which could be developed for printing on edibles with a bubblejet printer is extremely limited. The thermal jet printheads are also associated with ink drying on the surface of the jet channels. It would be preferable if piezojet systems could be developed for printing on edibles, as piezojet printheads do not require large temperature oscillations. Bubblejet printers are also much too slow to permit high speed printing directly on edible substrates. Thermal jet printheads (or bubble jet printheads, as they are also known) are used for transfer sheet applications, because the transfer sheets are porous and hydrophilic enough to be used with the known inks.
Water-based edible inks known to be thermal jet compatible are disclosed in co-pending U.S. application Ser. No. 10/211,592. However, it has been demonstrated that these inks are incompatible with hydrophobic surfaces.
A method of ink-jet printing on edibles is described in co-pending U.S. patent application Ser. No. 09/587,108. The ink described therein is a pigmented white ink which has found utility for printing on chocolate.
Pigmented ink compositions, including non-white colored pigmented inks, have been disclosed in the prior art, including the aforesaid U.S. Pat. No. 5,800,601. However, pigmented inks are less preferred for drop-on-demand ink-jet systems, because the pigments can influence the rheology of the ink causing poor jettability. Also, the presence of pigment particles may require additional maintenance of the printhead. Once printed, the pigment particles tend to sit on the substrate surface, flaking off easily, or requiring the use of additional binders and/or resulting in poor image adhesion.
Piezojet printheads are characterized by a piezoelectric device which causes ejection of ink from a reservoir, such as by a piezoelectric crystal deforming the wall of the reservoir in accordance with data signals. Heretofore, edible inks have not been developed for these systems. Piezojet printheads have larger channels than bubblejet printheads which affords a greater range of acceptable ink ingredients.
The food colorants approved for human consumption by the United States Food and Drug Administration, and natural colorants which generally do not require such approval, are water soluble, and consequently food grade inks made from such colorants are water-based. These water-based colorants tend to be poorly compatible with hydrophobic surfaces of M&M's® Milk Chocolate and Peanut Chocolate Candies, which are finished with carnauba wax.
In summary, the rotogravure technology is not capable of printing in full color on edibles and prohibits rapid changeover of print designs, while continuous jet printing is not capable of achieving high resolution. It would be desirable to print on edibles with piezojet ink-jet systems to achieve these objectives. However, water-based, non-pigmented inks suitable for piezojet printheads, particularly inks having compatibility with hydrophobic or waxy surfaces, have not heretofore been available. There is also a need to print higher resolution, full color images on confectionery, which rotogravure techniques are not capable of delivering. Rotogravure techniques, because they rely on contacting the substrate, can only print on a limited surface area of the substrate. The contact printing systems, including rotogravure, cannot accommodate irregularities in the size and shape of an edible printing substrate, such are found, for example, on M&M's® Milk Chocolate and Peanut Chocolate Candies.
Thus, there continues to be a need in the industry for edible ink-jet compatible inks, particularly piezojet compatible inks, which can be used to print high resolution images directly on edible substrates, particularly those edible substrates that are difficult to print on using conventional technology, such as those having a sugar shell surface with a wax or fat polish coating.